This invention relates to WDM (wavelength division multiplex) optical networks, and is particularly concerned with optical add/drop multiplexing (OADM) in optical WDM networks.
In this specification, as is common in the art, terms such as optical add/drop multiplex and OADM are used to embrace not only dropping one or more optical channels from and adding one or more optical channels to an optical signal, but also dropping one or more optical channels without adding any channels, and adding one or more optical channels without dropping any channels, and such terms should be understood and interpreted accordingly.
In addition, although in this specification for clarity and convenience optical signal directions on respective optical paths are described and illustrated, it should be understood that each optical path (e.g. an optical fiber) may simultaneously carry optical signals in opposite directions. Furthermore, optical components may simultaneously operate in opposite manner for optical signals in opposite directions; for example, an optical component which operates as a multiplexer for optical signals in one direction may simultaneously operate as a demultiplexer for optical signals in the opposite direction. Again, the following description is to be understood accordingly.
Optical WDM networks are known in which two or more optical channels are carried on a single optical fiber, each channel comprising an optical signal at a respective wavelength. At any node in the network, it may be desired to terminate one or more of the channels, for which purpose it is known to provide an OADM. An OADM typically comprises one or more optical channel filters and/or one or more optical band filters, where an optical band comprises a plurality of optical channels to be dropped or added. Optical channel and band filters are well known in the art and need not be described here.
The use of OADMs to drop and add individual optical channels or bands provides the advantage that the node-to-node optical connectivity of the network can be different from the physical connectivity of the optical fibers used to carry the channels. For example, the optical fibers may extend between adjacent nodes of a network, whereas the optical connectivity can be such that nodes can be selectively bypassed by some channels, depending upon the optical filters provided at the nodes. Consequently, in known optical networks the optical filters that are provided at each node are typically customized for that node and different from the optical filters provided at other nodes.
It thus becomes necessary to provide a substantial inventory of different optical filters for use in the different nodes, and to keep track of the optical filters that are used in each node. Furthermore, changes at the nodes, for example to drop and add additional or different optical channels, are complicated by the need for customized optical filters for each node.
Consequently, there is a need to provide an OADM and optical network which can enable different nodes to use optical components which are substantially the same for the different nodes and are independent of the particular optical channels which are to be dropped or added at each individual node.
According to one aspect of this invention there is provided an optical multiplexing arrangement for a WDM (wavelength division multiplex) optical network, comprising: a first optical path for receiving from a remote source a plurality of unmodulated WDM optical carriers at wavelengths of optical channels of the WDM network; a modulator arranged to modulate a signal to be transmitted onto at least a selected one of said WDM optical carriers; and a tunable optical add filter arranged to add to a WDM optical signal on a second optical path said selected one of said plurality of WDM optical carriers modulated with said signal to be transmitted.
Preferably the optical channels comprise groups of optical channels each comprising a respective optical channel in each of a plurality of optical bands, and each optical band comprises a plurality of optical channels having adjacent wavelengths. In one form of the invention the arrangement includes a tuneable drop filter and demultiplexer arranged to select said selected one of said plurality of WDM optical carriers for modulation by said modulator. In a preferred form of the invention the modulator is arranged to modulate said signal to be transmitted onto all of the optical carriers having wavelengths in a respective optical band, and the tunable optical add filter is arranged to select for adding to the WDM signal on the second optical path an optical channel in each of the optical bands.
The invention also provides a WDM optical network comprising a plurality of optical multiplexing arrangements each as recited above optically coupled via respective ones of said second optical paths, and a source of said plurality of unmodulated WDM optical carriers coupled via said first optical path to each optical multiplexing arrangement.
Another aspect of the invention provides an optical add/drop multiplexer (OADM) comprising: an optical path for receiving from a remote source a plurality of unmodulated wavelength division multiplex (WDM) optical carriers at wavelengths of optical channels of a WDM optical network; a modulator arrangement arranged to modulate at least a selected one of said unmodulated optical carriers with a signal to be transmitted to produce an optical signal channel having a selected wavelength; and a tunable optical add filter arranged to add said optical signal channel having said selected wavelength to a WDM optical signal path.
The modulator arrangement can include a tuneable drop filter and demultiplexer arranged to select said selected one of said unmodulated optical carriers for modulation with said signal to be transmitted. Alternatively, the modulator arrangement can comprise a demultiplexer arranged to demultiplex the optical carriers into a plurality of optical bands each comprising a plurality of optical carriers having adjacent wavelengths; one or more modulators each arranged to modulate the plurality of optical carriers of a respective optical band with a respective signal to be transmitted; and a multiplexer arranged to multiplex outputs of the modulators for supply to the optical add filter; the optical add filter being arranged for adding to the WDM optical signal path a group of optical signal channels comprising one optical signal channel having a selected wavelength from each of said optical bands.
The invention further provides a WDM optical network comprising a plurality of OADMs each as recited above optically coupled via the WDM optical signal path, and a source of said plurality of unmodulated WDM optical carriers coupled via said optical path to each OADM.
A further aspect of the invention comprises a method of providing, at each of a plurality of nodes in a wavelength division multiplex (WDM) optical network, an optical add/drop multiplexer (OADM) using optical components which are not dependent upon particular wavelengths of optical channels to be dropped or added at the respective node, comprising the steps of: grouping optical channels into a plurality of groups each comprising a respective optical channel in each of a plurality of optical bands, each optical band comprising a plurality of optical channels having adjacent wavelengths; providing in each OADM at least one tunable optical filter arranged to drop from and/or add to an optical signal path of the network optical channels in a selected one of said groups; supplying from a remote source via an optical carrier path to OADMs in different nodes a plurality of unmodulated optical carriers at wavelengths of optical channels of the network; and in one or more OADMs in which an optical channel is to be added to the optical signal path, modulating at least one of the unmodulated optical carriers having wavelengths in a respective optical band with a signal to be transmitted.
The invention further provides a method of providing unmodulated optical carriers at wavelengths of WDM optical channels to a plurality of nodes in a WDM optical network for use in carrying out the method recited above, comprising the steps of: producing said unmodulated optical carriers at an optical carrier source remote from at least some of said nodes; and distributing said unmodulated optical carriers from the optical carrier source via the optical carrier path to each of said nodes remote from the optical carrier source.